Guanylin and uroguanylin are peptides that are expressed primarily in the mammalian intestine and were identified because of their homology to the bacterial heat stable enterotoxin (ST) and their ability to bind to the ST receptor, guanylyl cyclase C (GC-C). ST is a worldwide cause of secretory diarrhea whereas guanylin and uroguanylin are thought to modulate intestinal secretion without causing diarrhea. Therefore, a better understanding of the mechanisms and receptors through which these peptides act will likely shed light on the pathophysiology of toxigenic diarrhea. To further explore the physiologic role of guanylin and uroguanylin we created guanylin and uroguanylin gene-targeted mice. We found in preliminary experiments that uroguanylin has an important regulatory role in salt homeostasis. Therefore, the overarching hypothesis of this application is that uroguanylin and guanylin are enteric hormones forming both a local paracrine regulatory system in the intestine and a hormonal system as part of an enteric-renal axis to regulate natriuresis. We will address the following specific aims. I) Define the role of guanylin and uroguanylin as endocrine hormones regulating salt and water metabolism in gene targeted mice. We will test the hypotheses that: 1) Uroguanylin but not guanylin or GC-C deficient mice, will demonstrate impaired urinary sodium and potassium excretion. 2) Uroguanylin deficient mice will demonstrate salt sensitive hypertension. 3) These actions of uroguanylin are mediated via a novel, non-GC-C receptor. II) Determine the mechanisms regulating guanylin and uroguanylin expression. We will test the hypotheses that: 1) Basal levels of guanylin and uroguanylin expression are regulated by protein kinase C (PKC) isoforms. 2) Hypertonicity upregulates guanylin and uroguanylin by a transcriptionally-regulated, PKC-dependent mechanism. 3. Interferon-gamma (IFN-gamma) downregulates guanylin and uroguanylin via a transcriptionally-regulated mechanism. III) Define the cellular secretory defects resulting from uroguanylin loss in intestine and kidney. We will test the hypotheses that: 1) Loss of uroguanylin will result in impaired intestinal secretion of chloride. 2) In the kidney, loss of uroguanylin will result in dysregulated water and solute transport in proximal tubular cells leading to intracellular vacuole formation. 3) Loss of uroguanylin will result in compensatory upregulation of novel uroguanylin receptor gene(s) and compensatory counter-regulation of transporters and water channels expressed in enterocytes in the proximal small intestine and proximal tubule cells in the kidney. These studies will further delineate the role and action of these peptides and ST in the intestine as well as their roles as intestinal hormones in an enteric-renal axis. Furthermore, these studies will lay the groundwork for translational research involving these peptides as therapeutic agents for fluid overload states.